A down-scale and higher integration degree of the semiconductor devices has required a technique for realizing fine circuit patterns. For this, there has been developed several studies including improvement of an exposure instrument or pattern forming method. The improvement of the exposure instrument has some drawbacks such as requiring considerable initial investment and reducing utilization of a conventional instrument. Thus, it is more preferred to improve the pattern forming method.
A directed self assembly (DSA) lithography using a self-assembly of a block copolymer (BCP), as one of improved pattern forming method, is expected to be capable of forming fine patterns having a line width of 20 nm and less than, which is known as the limit of a conventional optical pattern forming method. The DSA lithography, in which the conventional photoresist pattern process and an orientation characteristics of BCP are integrated, makes BCP oriented in a certain direction, thereby forming the fine resist patterns. In detail, in the DSA lithography, after forming the photoresist patterns (guide patterns) on a wafer or a thin film of ITO glass by using a conventional ArF, KrF or I-line photoresist composition, BCP is coated on spaces between the photoresist patterns and then heated to form the BCP coating layer. The BCP coating layer is subject to a heating treatment at a temperature over a glass transition temperature (Tg) and is rearranged (self-assembled or self-oriented). A part of the rearranged BCP is removed to obtain a self-assembled resist pattern with an ordered orientation (Please refer to Korean patent unexamined publication No. 10-2010-0126190, Korean patent application No. 1 0-201 1-0098838 filed on Sep. 29, 2011).
Generally an underlayer of the BCP should be a neutral layer in order to form fine patterns by the DSA lithography. Silicon wafers or ITO glasses used in a semiconductor process or LCD process have different polarity according to the materials thereof, and would hinder the self-assembly of the BCP, thereby making the formation of patterns difficult. For example, in case when the resist underlayer film is of nonpolar materials, a nonpolar part of BCP is adjacent to the resist underlayer film, and in case when in case when the resist underlayer film is of polar materials, a polar part of BCP is adjacent to the resist underlayer film. As a result, while a desirable lamella structure of perpendicular orientation is not formed, a lamella structure of parallel orientation is formed. Accordingly, it is necessary to form the neutral layer under the BCP for obtaining the desired lamella structure of perpendicularly oriented BCP.
For forming such neutral layer, hydroxyl terminated PS-r-PMMA(polystyrene-r-poly(methylmethacylate)) random copolymer, styrene homopolymer, or random copolymer of styrene and epoxy etc. was conventionally used as a compound having medium polarity between the polar part and the nonpolar part of BCP. When the neutral layer is made of PS-r-PMMA, the perpendicular orientation of BCP is possible, but the resist underlayer film should be silicon and as a drawback, high heating for several hours is required so as to form a mono layer of hydroxyl terminated PS-r-PMMA. Further, since the conventional neutral layer has low refractive index of about 1. 7, it is not proper as the resist underlayer film in a photoresist forming step using ArF exposure instrument. Accordingly, there is carried out a complicate process that either a neutral layer of 10 nm is spin-coated on a conventional antireflection coating (ARC) layer of 33 nm (in this case, the total thickness of the resist underlayer film is 43 nm), or a neutral layer is spin-coated on an antireflection coating (ARC) layer containing silicon, and then unreacted part of the neural layer is removed by dissolving in an organic solvent. Like this, in case of grapho-epitaxy method, the ARC layer must be additionally formed under the neutral layer.